Courier IMAP server will not respond.

I have just moved from a Fedora server using Dovecot, to Debian 6 with Courier.
I'm having a problem connecting to IMAP on port 993. The server does not respond. I've tried everything I can think of, and I still cannot connect, so i'm hoping somebody can shed some light on this for me.

The server was configuring using the Debian 6 ISPConfig3 Perfect Server setup.
There are no useful errors in mail.log, only entries like these below: